In order to reliably read data from or write data onto a disk media surface of a disk drive, a read/write transducer or head must be positioned precisely over a track of the media surface from which data is read or on which data is written. Failure to accurately position the read/write head over the desired track during a read operation results in unreliable data retrieval. If the read/write head is improperly positioned during a write operation to the disk, not only may the written data be lost, but data on adjacent tracks may be written over and destroyed. The accurate placement of the read/write head is therefore crucial to the utility of disk systems.
One method used for head position control is a disk drive servo system. The disk drive servo system typically utilizes a head positioning actuator assembly for precisely positioning the head over the desired data track. The head actuator assembly includes an actuator motor which operates to position the head under the control of a head actuator servo loop. The head actuator control servo loop both initially positions the read/write heads over the desired data track and maintains the heads in that position.
The head actuator assembly and disks are typically housed in a Head Disk Assembly ("HDA"). The HDA is typically mounted to a drive chassis with the use of a plurality of shock mounts typically fabricated of rubber or other suitable compliant material. The stiffness of the shock mounts and other physical characteristics such as the inertia of the HDA determine the absorption or attenuation of shock forces applied to the HDA and to the drive chassis. This absorption or attenuation is practically limited due to geometrical constraints on HDA and shock mount size.
The application of physical vibration or shock to the drive chassis or internally generated vibration or shock to the HDA itself tends to cause positioning errors in the servo system. These disturbances may be introduced by, for example, spindle imbalance forces, external shock and vibration, as well as a self-induced shock mount/HDA resonance excitation known to those skilled in the art as windup. For example, reaction forces applied to the HDA by the actuator motor during seek activity may cause windup.
In known systems, a tradeoff exists between sensitivity to external shock and vibration on the one hand and to windup on the other. Higher compliance shock mounts may provide less sensitivity to chassis vibration but lower the HDA/shock mount resonant frequency and increase sensitivity to windup. The opposite is usually true also.
Known disk drive head positioning servo systems typically use a linear actuator assembly. This type of assembly utilizes a guided carriage arm assembly having a plurality of heads supported at one end and a voice coil at the other end with supporting bearings positioned between the voice coil and the heads. Other disk drive head positioning servo systems use an unbalanced rotary actuator. In these systems, the unbalanced rotary actuator may be a center-of-percussion type. In both such configurations, linear acceleration component sensing provides compensation for shock and vibration disturbances.
In the case of servo systems with a balanced rotary actuator, however, translational or linear HDA acceleration measurement accomplished with a single linear accelerometer cannot sense angular acceleration of the HDA without being sensitive to other HDA vibration components. Such components include both pure translational and other angular vibration components. Accordingly, known compensation or disturbance rejection systems, while performing satisfactorily for applications using linear or unbalanced rotary actuators, fail to address the problems of spindle imbalance forces, external shock or vibration and windup in systems having a balanced rotary actuator. This failure is due to the fact that only angular acceleration of the HDA in the direction of actuator rotation substantially causes positioning errors in systems that utilize a balanced rotary actuator. Other HDA acceleration components do not substantially induce such errors. Sensing these other components to provide compensation to the head positioning servo system typically induces positioning errors and, in fact, increases the sensitivity of the servo system to shock, vibration and windup.